"He can move in it, which is so important. You can tell he's ready to move," Duff tells contestant Sonjia Williams of her look for a flirty toddler who bats his lashes at the actress. "I love that you can dress it up or down, and I love what mom is wearing as well." (Duff has her own clothing line, Stuff by Hilary Duff.)

Agrees mom of four Klum of Williams' pint-sized client: "He looks like a little man!"

Welcoming baby Luca in March with husband Mike Comrie, Duff is adjusting well to first-time motherhood. "It's really wonderful," she told Us Weekly in August of watching her little boy grow. "He's getting his first teeth and he's rolling over like a maniac. [Motherhood] really the best ever."

Watch more of Duff's appearance on Project Runway in the clip above before the full episode -- which also features the contestants caring for babies of their own -- airs Thursday at 9 p.m. (EST) on Lifetime.